68 
SEA-SHELLS AND THEIR MAKERS. 
bottom or prolonged into a shelly canal for the protection 
of the siphon. This canal is formed by a prolongation of 
the columella and of the outer lip of the aperture, and is 
sometimes as long as or even longer than the spire, so that 
the shell is fusiform or spindle-shaped. One such shell, 
of which several species occur in the Indian Ocean, is 
called Fusus, which is the Latin for a “ spindle ”; another is 
known as Fasciolaria; Murex and Turbinella are also 
shells with long canals. 
In other cases the whole shell is coiled up on its axis, 
and is then said to be convolute, the body-whorl being very 
large and the aperture a long narrow slit. The Cowries 
(Cyprcea ), so well known as ornamental shells, are an 
instance, but they are spiral when young, and only become 
convolute at a certain age. The Olive-shells {Oliva) are 
partially convolute, having a very short spire and a cylin¬ 
drical body-whorl with a narrow aperture. 
The mention of Cowries and Olives leads me to say a 
few words about the outer surface of shells, why some are 
smooth and even polished, others rough and ribbed, and 
others, again, tubercular, spiny, or frondose. The surface- 
layer of the shell is produced by the outer margins of the 
mantle or body-covering, and these margins are either 
smooth, rugose, or elongated into projections in accord¬ 
ance with the kind of surface which is produced, and which 
we must suppose is in some way beneficial to the animal. 
In some cases the borders of the mantle are reflected 
over the shell, sometimes to such an extent that they cover 
nearly the whole of it, and in such cases the shell generally 
has quite a polished surface, as in Cyprcea, Oliva, and Natica. 
But others produce shells with a similar natural polish, 
without any reflection of the mantle; such are the Auger- 
shells ( Terebra ), the Pheasant-shell {Phasianella) , and 
Eulima. Again, there is at least one genus where the 
mantle is reflected over the shell, and yet that is rough, 
with a raised reticulate ornamentation; this genus is 
Pirula. Lastly, there are cases where some species of a 
genus are smooth and polished though the majority are 
ribbed or striated, such a contrast being found in two 
Bornean shells, Nerita polita and Nerita lineata. Hence it 
is clear that there is no relation between the smoothness 
of a shell and the reflection of its maker’s mantle over its 
edges; it is evident that the smoothness or roughness of 
the shell is co-ordinated with some requirement of the 
animal’s environment about which at present we know 
